It obviously explodes. I believe we have a topic for questions like these. I'd believe that they'd pack up beforehand and find another system to live in, though.

“Live a good life. If there are gods and they are just, then they will not care how devout you have been, but will welcome you based on the virtues you have lived by. If there are gods, but unjust, then you should not want to worship them. If there are no gods, then you will be gone, but will have lived a noble life that will live on in the memories of your loved ones.” - Marcus Aurelius

The Mahlok would try to harness/harvest the power of the exploding sun and stop it from destroying their empire. Then may then use that harnessed energy to beat down the other factions. That might make an interesting story actually...

Anyway, or they could use it to destroy the Skrakki things once and for all. The human factions or Harmony might try to do the same. It could lead to a war of harvest the suns energy and using it as weapons in order to capture more energy.#

But, as other people have said, there is always more terraformed planets to travel to.

The Mahlok would try to harness/harvest the power of the exploding sun and stop it from destroying their empire. Then may then use that harnessed energy to beat down the other factions. That might make an interesting story actually...

Anyway, or they could use it to destroy the Skrakki things once and for all. The human factions or Harmony might try to do the same. It could lead to a war of harvest the suns energy and using it as weapons in order to capture more energy.#

But, as other people have said, there is always more terraformed planets to travel to.